1. Field of the Invention
The present invention relates to a digital television tuner for demodulating a digital television signal which is transmitted in the form of what is commonly called a terrestrial wave and for outputting a baseband signal.
2. Description of the Related Art
FIG. 4 is a diagram of a representative digital television tuner of a direct conversion method, which has hitherto been conceived by the inventor of the present invention. A digital television signal (hereinafter referred to simply as a xe2x80x9ctelevision signalxe2x80x9d) of a channel (for example, a U.S. television channel) assigned between about 55 MHz and 806 MHz is input to an input end 41. The band of one channel is 6 MHz in a manner similar to the band of an existing analog television. The input television signal first passes through a baseband filter 42 and then a wide-band low-noise amplifier 43 and is input to two first mixers 44 and 45.
A first local oscillation signal for demodulation, which is output from a first local oscillator 46, is input to the first mixers 44 and 45. The frequency of the first local oscillation signal is the same as the center frequency of the band of a channel to be received, and the signal is input to one of the mixers (for example, the mixer 45) in such a manner as to be shifted in phase by 90xc2x0 C. by a phase shifter 47. As a result, two baseband signals (I signal and Q signal) whose phases are mutually in an orthogonal relationship are output from the first mixers 44 and 45. Then, since the frequency of the first local oscillation signal is the same as the center frequency of the channel to be received, the baseband signals are output at a band of 3 MHz, which is almost half in a state in which one half of that band is folded around the center frequency.
One of the baseband signals (I signal) passes through a low-pass filter 48 and is input to an A/D (analog-to-digital) converter 49 whereby the signal is converted into a digital signal (called a digital I signal, and is referred to as a xe2x80x9cDI signalxe2x80x9d in FIG. 4), and the other (Q signal) passes through a low-pass filter 50 and is input to an A/D converter 51 whereby the signal is converted into a digital signal (called a digital Q signal, and is referred to as a xe2x80x9cDQ signalxe2x80x9d in FIG. 4). The frequency components of the digital I signal and the digital Q signal are distributed up to about a dozen MHz, although it depends on the sampling frequency at the time of digital conversion.
The first mixers 44 and 45 cover a wide frequency range (55 MHz to 806 MHz). For this reason, an imbalance occurs between the respective phases, gains, etc., of the I signal and the Q signal depending on the received channel. This imbalance is corrected by using one of them (for example, the digital Q signal in FIG. 4) as a reference and by digitally processing the phase, gain, etc., of the other (the digital I signal). As a construction for correcting, an orthogonal correction circuit 52, a second mixer 53, and a first adder 54 are provided.
Then, the digital Q signal is input to the orthogonal correction circuit 52 and the second mixer 53, and the digital I signal is input to the first adder 54. Also, the digital I signal whose phase, gain, etc., have been corrected is input to the orthogonal correction circuit 52. The orthogonal correction circuit 52 detects information to be corrected, such as the phase, the gain, etc., from the digital Q signal and the corrected digital I signal, and inputs it to the second mixer 53, whereby correction information is superimposed on the digital Q signal. Then, the digital Q signal on which the correction information is superimposed is input to the first adder 54, and the phase, the gain, etc., of the digital I signal are corrected.
The corrected digital I signal and the digital Q signal are input to third mixers 55 and 56, respectively, whereby they are mixed with a second local oscillation signal from a second local oscillator 57 and a phase shifter 58, the original band is widened again and, thereafter, is added by a second adder 59 and is output.
In the manner as described above, in the conventional construction, in order to obtain a baseband signal by directly demodulating a television signal of a received channel, a local oscillation signal (first local oscillation signal) having the same frequency as the center frequency of the band of the received channel is used. For this reason, there is a problem in that this local oscillation signal leaks to the input end 41 side. For the leakage of the local oscillation signal, only the reverse-direction transmission characteristics of the low-noise amplifier 43 function, but these are not sufficient to stop the leakage.
Also, since the band of the demodulated baseband signal (I and Q signals) becomes one half as large, a construction (the second local oscillator 57, the third mixers 55 and 56, and the phase shifter 58) for restoring the original band is required. Furthermore, since the first mixers 44 and 45 cover a channel of a wide band, an imbalance occurs in the performance between the two baseband signals, and a construction (the orthogonal correction circuit 52, the second mixer 53, and the first adder 54) for correcting this imbalance is required. Therefore, there is a problem in that the construction of the portion where the baseband signal is processed becomes complex.
Accordingly, an object of the present invention is to realize a digital television tuner which is simple in the construction of a baseband signal processing portion, in which it is difficult for a local oscillation signal to leak to the input end side, and in which image interference is suppressed.
To achieve the above-mentioned object, according to the present invention, there is provided a digital television tuner comprising a frequency conversion unit for converting the frequency of a digital television signal of a channel to be received from among channels arranged in a predetermined frequency band into the highest frequency of the frequency band or higher; two mixers for outputting two baseband signals whose phases are mutually orthogonal to each other by demodulating the frequency-converted digital television signal; a local oscillator for inputting local oscillation signals whose phases are mutually orthogonal to each other to the two mixers; and an addition unit for adding together the two baseband signals, wherein the frequency of the local oscillation signals is set to be the same frequency as the frequency at the end of the band in the channel of the frequency-converted digital television signal, and one of the baseband signals is converted into the same phase as the other, after which the signals are input to the addition unit.
Also, the digital television tuner of the present invention generates a local oscillation signal by using a pilot signal superimposed on the end of the band of the channel to be received.
Also, in the digital television tuner of the present invention, a Hilbert filter is provided between one of the mixers and the addition unit, and one of the baseband signals is input to the addition unit via the Hilbert filter.
Also, in the digital television tuner of the present invention, an analog-to-digital converter for converting the baseband signal into a digital signal is provided between each of the two mixers and the addition unit, and the Hilbert filter is provided between one of the analog-to-digital converters and the addition unit.
In the digital television tuner of the present invention, the frequency of the frequency-converted television signal is set to approximately 1 GHz.
The above and further objects, aspects and novel features of the invention will become more fully apparent from the following detailed description when read in conjunction with the accompanying drawings.